1. Field of the Invention
The invention is related to the field of measurement of structural attitude ("dip") of earth formations. More specifically, the invention is related to methods for processing data from "imaging" well logging instruments, for calculating the structural attitudes of the earth formations.
2. Description of the Related Art
Well logging instruments used to determine the structural attitude ("dip") of earth formations are of two general types. First are the so-called "diplog" or "dipmeter" instruments which measure formation resistivity to very fine detail (high axial resolution) along the wall of a wellbore drilled through the earth formations. The resistivity is measured at three or more circumferentially spaced apart positions around the wellbore. Differences in the axial position, from one of the circumferentially spaced apart position to the others, of any changes in value of the resistivity are calculated from the measurements. The differences in the axial positions of changes in the resistivity values are used to calculate the dip of the earth formations. The assumption being made in these calculations is that changes in resistivity values correspond directly to different depositional layers of the earth formations. Representative methods for calculating dip are shown in U.S. Pat. No. 4,414,656 issued to Hepp and U.S. Pat. No. 4,453,219 issued to Clavier et al.
More recently, well logging instruments have been developed which make even more finely detailed measurements of resistivity. These more recent instruments, known in the art as "imaging" devices, make measurements which cover the circumference of the wellbore substantially continuously. The measurements of resistivity can be converted to an optical (visual) presentation where color or grey scale density corresponds to the resistivity, so that the visual presentation, or "image", of the wellbore wall can be obtained. See for example U.S. Pat. No. 4,567,759 issued to Ekstrom et al.
Another type of "imaging" instrument, known as an ultrasonic acoustic imaging instrument, uses reflectance amplitude and travel time of ultrasonic energy to generate an image of the wellbore wall. The acoustic imaging instrument has been also combined with the resistivity instrument into a single well logging tool. See for example U.S. Pat. No. 5,502,686 issued to Dory et al.
Irrespective of the type of imaging instrument used, dip calculations made from the image data are made in substantially the same manner as for the diplog instruments. It has become apparent from the visual representations made by imaging instruments that the prior art methods for calculating dip do not satisfactorily resolve dip when there are both "structural" and "stratigraphic" components to the dip in certain earth formations. The term structural dip describes the overall attitude of a particular stratigraphic unit or layer. It is generally believed that sedimentary earth formations, which are the kind most commonly measured by well logging instruments, were originally deposited horizontally. As large portions of the earth's crust underwent structural movement, the overall attitude of these layers could have been shifted from horizontal. The change in attitude results in the structural dip. Sometimes the forces which move the large sections of the earth's crust cause the sedimentary layers to bend or "fold" as well as shift from horizontal. Some components of dip can be associated with folding as well as gross overall change in attitude. Sedimentary earth formations also can have layering features related to the environment in which the sediments were deposited. These features can impart significant attitude changes within a single earth formation layer. These intra-layer attitudes are generally known as stratigraphic dip. It has proven difficult using prior art methods of dip calculation to resolve stratigraphic dip where a structural dip is also imposed on the same earth formations or where fold-related dip is superimposed on structural and stratigraphic dips.